Design advances in high-performance electrochemical systems such as fuel cells, sensors, and the like depend on the availability of polymer materials that exhibit high ionic conductivity and chemical and thermal stability, as well as good mechanical strength characteristics, over a wide range of operating temperatures. Among the variety of materials used for these purposes, ionogenic polymers (ionomers) are considered suitable polymer matrices for proton-conducting membranes and electrode materials.
Ionomers are important components of semi-permeable membranes, which allow specific particles (in this case ion species) to pass through while retaining others. Membranes made with ionomers can be fairly ion-specific or ion-selective. Ionomer-based materials of most interest for electrochemical applications such as fuel cells are those that selectively pass hydrogen ions (protons).
An important ion-selective membrane material in current use as a fuel cell membrane material is a perfluorosulfonate ionomer made by DuPont under the brand name Nafion®. Such perfluorosulfonate ionomers have adequate thermal stability up to approximately 90° C., and the ability to absorb adequate amounts of water. These perfluorosulfonated polymers (Nafion-type) are the most widely used proton-conducting materials for fuel cell membranes. However, their high cost and limited operational temperature range are significant disadvantages, especially at higher fuel cell operating temperatures. The thermal stability of perfluorosulfonated polymers is insufficient above 90° C. For example, the lifetime of Nafion 117 at 120° C. is only 30-45 days. The disadvantages of high cost and low stability at higher temperatures severely limit the application of these materials to advanced (i.e. low cost and high temperatures) electrochemical systems.
For this reason, there has been increased interest in development of new non-fluorinated proton conducting materials that cost less and perform better as fuel cell membranes than does Nafion. Such non-fluorinated materials have been shown capable of operating in fuel cells at temperatures in excess of 120° C. Above this temperature, water evaporation from the membrane leads to a dramatic drop in conductivity.
There are a number of reasons that higher temperature ion-selective membranes would be of value in electrochemical applications such as fuel cells. The first is that kinetic rates of electrochemical reactions generally increase with increasing temperature, resulting in improved fuel cell performance. Another advantage of high temperature operation (˜150° C.) is that Pt catalyst poisoning by trace amounts of CO, which can be present in hydrogen fuels, is greatly reduced or eliminated.
In summary, the desired characteristics of membranes for fuel cells for both mobile and stationary applications are as follows:                1. Operation at a temperatures of 120-150° C.,        2. Low resistance (high proton conductivity) under cell operating conditions,        3. Long-term chemical and mechanical stability at elevated temperatures in oxidizing and reducing environments,        4. Good mechanical strength, preferably with resistance to swelling,        5. Low gas cross-over (pinhole free),        6. Interfacial compatibility with catalyst layers,        7. Low cost,        8. Minimal or zero dependence on tightly controlled humidity.        